This invention concerns a method of heat treating a thin sheet coated with a zinc alloy containing aluminum by hot dip galvanization.
Hot dip galvanizing of a thin sheet with zinc and/or aluminum or alloys thereof in a continuous operation is usually performed by passing the strip through a bath of the molten coating material. The surfaces of the strip are wetted with the coating material. After the coated strip leaves the hot dip treatment bath, excess molten coating material is blown off the surface of the strip. The molten coating material cools down and solidifies in the process. While still hot, the coated strip is then either cooled to room temperature or subjected to another treatment. For example, hot dip galvanized strip is subjected to an aging treatment. To do so, the strip is annealed for approximately three minutes at a temperature in the range of about 350.degree. C. to improve its deep drawability (Stahl und Eisen [Steel and Iron], volume 102 (1982) no. 24, page 1236).
To improve the surface quality of an enamel or paint layer applied to a galvanized or aluminized thin sheet, it has already been proposed (European Patent No. 710,732 A1) that the coated strip be heated to a temperature above 300.degree. C. up to a temperature below the alloying temperature for less than five minutes. This should prevent micropores from forming in the enamel or paint layer.
The ZnAl melt used according to this invention is a ZnAl alloy containing 3.5-15% Al as the main ingredient in addition to zinc plus traces of rare earths. In addition, traces of magnesium, manganese, copper or silicon may also be present.
The composition of the ZnAl melt yields a solidification behavior leading to "dented" grain boundaries ("grain boundary dents") in the coating surface. This grain boundary formation has a negative effect on the surface appearance. Thus, use of this material in applications where high demands are made of surface quality is limited. Such applications include household appliances and automotive body parts which are provided with a high-quality enamel coating after being shaped and joined.
The object of the present invention is to improve upon the surface quality of the ZnAl coating so that a high-quality surface is obtained after cold working in combination with enameling or other coating methods (chromatizing, phosphatizing, protective enamel coating).
This invention will attempt to create an expedient here and provides for the thin sheet to be heated to a temperature 20 to 100.degree. C. above the melting point of the coating material for two to ten seconds after solidification of the coating applied to its surfaces in the hot dip galvanizing bath and then to be cooled to room temperature.
A significant improvement in surface appearance due to a reduction in grain boundary depth, which is very marked in ZnAl coatings without the use of the heat aftertreatment according to this invention, is found on the finished thin sheet with the heat-treated coating according to this invention. The bloom structure which would otherwise appear is macroscopically blurred and cannot be detected even after enameling or painting.
This macroscopically detectable change is associated with a microscopic change in the structure of the ZnAl coating. In the starting condition, the typical Zn +5% Al coating consists of the .gamma.-mixed crystal and a eutectic of .eta.- plus .beta.-phases.
After the annealing treatment according to this invention, there is a significant change. The original coarse .eta.-mixed crystal areas are now finely distributed and very numerous. The grains present at the ZnAl surface develop anew and with a significantly smaller grain size due to the heat treatment.
According to a preferred embodiment of the method according to this invention, use for thin sheets coated with a coating of a zinc base alloy containing 4.5 to 5.5% Al is therefore proposed.
A preferred heating is performed according to this invention by an electroinductive method. This permits very precise regulation of the temperature and duration of heating.
Another object of this invention is to improve the workability of a thin sheet coated with a zinc base alloy containing aluminum in a hot dip galvanizing process in such a way that development of cracks in the forming operation is prevented. It is known that thin sheets coated by hot dip galvanizing tend to develop cracks. In the past, this problem has not been solved satisfactorily.
The problem described here is solved by the heat treatment according to this invention. Thin sheets coated by hot dip galvanization and subsequently heat-treated in this way develop considerably fewer and smaller cracks after forming.
Heat-treated thin sheets coated by hot dip galvanizing by the method according to the present invention are suitable especially for applications where high demands are made of surface quality. This is the case especially for household appliances and automotive body parts which are provided with a high-quality paint coating, chromatized coating, phosphatized coating, protective paint coating, enameling or similar surface coating after being shaped and joined.